Thermosetting urea molding compounds are conventionally prepared by impregnating a cellulosic filler such as alpha cellulose with a mixture of urea and formaldehyde or a urea-formaldehyde condensate or a urea-formaldehyde prepolymer or resin. The impregnated material may be then dried under conditions that increase the condensation of the resin to produce a fluffy material commonly referred to as "corn." The corn is then mixed or ground in combination with curing catalysts and normally pigments and sometimes plasticizers to form a mixture or powder. Lubricants may be added before or after drying. The mixture or powder is either used as such or, if a powder, more commonly densified by various techniques to form a granular product. An overview of such a process is given in Modern Plastics Encyclopedia, Vol. 45, No. 14A, pp 157,176 (October 1968). Alternatively, the mixture may be fed directly to an extruder or the like to form a granular material.
A common application of such filled urea molding compounds is in injection molding, particularly of electrical parts such as receptacles, sockets, switch parts and circuit breakers. In the normal injection molding process, the granular molding compound is rammed as a shot through a sprue and runners and through gates into a plurality of mold cavities. The mold cavities are subjected to sufficient heat and pressure to set the urea molding compound into the desired articles. Since the spure and runners are in thermal communication with the mold cavities, the molding compound in the sprue and runners also sets during the normal mold cycle. When the mold is opened, material from the mold cavities, the runners and the sprue must be ejected, with the sprue and runner material representing substantial waste of molding compound.
A "cold manifold" injection molding process has been developed for thermosetting materials such as phenolics wherein the manifold is kept from heating to the temperature where materials cure. Thus, only the molded articles and relatively short runner pieces need be ejected, with the remainder of the material in the sprue and runners forming a portion of the subsequent shot.
Exemplary equipment for such processes and an overview of the processes themselves are contained in U.S. Pat. No. 3,374,502 to Lazzara (issued Mar. 26, 1968), an article entitled "Cold-Manifold Molding Gains Adherents-Slowly" in the August 1977 issue of Plastics Technology on pages 55 and 56, an article entitled "Molders Speak Out On Cold-Manifold Molding" in the December 1977 issue of Plastics Technology on pages 55-59 and Pennwalt Corporation (Stokes Division) "Cold Runner Manifold," Plastics World 35-36 (September 1977). While such a cold manifold injection molding system has been suggested for urea molding compounds, a satisfactory urea molding compound for such use is not available. It should be recognized that very high molding temperatures are used to cure phenolic molding compounds, allowing the use of higher manifold temperatures than for urea molding compounds to achieve a satisfactory injection viscosity without experiencing setting or curing in the manifold. This permits the wide temperature difference between mold cavity temperature and manifold temperature thus facilitating the successful molding of phenolic materials in a "cold manifold" injection molding process.
The difficulties are that conventional urea molding compounds will set quickly in the sprue and runners at the temperatures required to inject or move the material through the manifold and runners. Such setting is particularly likely to occur if, because of operational problems, material remains in the runners for a time equivalent to several cycles. In attempting to vary urea molding compound compositions to avoid setting in the runners, the rate at which the compound sets or cures in the mold cavity may also increase such that cycle times must be substantially increased. Furthermore, with some other modifications in the formulation including sufficient catalyst to achieve reasonable cycle times, a satisfactory initial injection viscosity of the material can be achieved, but subsequent viscosity increases in the manifold cause injection difficulties.
It is accordingly an object of this invention to provide a urea molding compound that can be maintained at cold manifold temperatures in the range of about 180.degree. F. to 230.degree. F. (about 82.degree. C. to 110.degree. C.) without setting or curing for several minutes, and without excessive viscosity under injection conditions or unreasonably long molding cycle times and that can be prepared or manufactured by conventional methods without substantial cost penalty.